Flourish as learner in time of prosperity (5)
I believe the answer is:
bloom
'flourish' is the definition.
(I've seen this in another clue)
'learner in time of prosperity' is the wordplay.
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'in' indicates putting letters inside.
'time of prosperity' becomes 'boom' (I've seen this in other clues).
'l' put inside 'boom' is 'BLOOM'.
'as' acts as a link.
(Other definitions for bloom that I've seen before include "Coat (lens); flower" , "(Come into) flower" , "look good" , "Orlando" , "Blossom, flower" .)